Astronomy Quiz: Nebular Theory

20 questions · exam conditions

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Nebular TheoryQuestion 1 of 20

As a large, slowly rotating interstellar cloud collapses under its own gravity, the conservation of angular momentum causes its rotation to speed up and the cloud to flatten into a disk. Which of the following provides the most accurate physical explanation for why the cloud flattens?

The increased rotational speed generates an outward centrifugal force that most strongly opposes gravity along the rotational equator, allowing collapse to proceed more rapidly along the poles.

Gravitational forces are weakest along the rotational poles due to the cloud's spin, causing the cloud to collapse preferentially into a disk shape.

Inelastic collisions between infalling particles systematically cancel out motions perpendicular to the average plane of rotation, forcing material into a disk.

The central protostar's early solar wind exerts pressure that pushes material away from the poles more effectively than from the dense equatorial region.

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Astronomy Quiz

Astronomy Quiz: Nebular Theory

Practice Nebular Theory in Astronomy with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Nebular Theory, giving you a quick way to practice the rules, question types, and explanations that matter most for Astronomy.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

The increased rotational speed generates an outward centrifugal force that most strongly opposes gravity along the rotational equator, allowing collapse to proceed more rapidly along the poles. (correct answer)
Gravitational forces are weakest along the rotational poles due to the cloud's spin, causing the cloud to collapse preferentially into a disk shape.
Inelastic collisions between infalling particles systematically cancel out motions perpendicular to the average plane of rotation, forcing material into a disk.
The central protostar's early solar wind exerts pressure that pushes material away from the poles more effectively than from the dense equatorial region.

Explanation: The correct answer is A. As the cloud spins faster, the centrifugal force (or inertial effect in a non-rotating frame) counteracts gravity. This effect is strongest at the equator (perpendicular to the axis of rotation) and non-existent at the poles. Therefore, gravity can pull material in along the poles much more easily than it can pull material in at the equator, resulting in a flattened disk structure. Choice C describes a real and contributing process (inelastic collisions), but the primary driver of the overall flattened shape is the balance between gravity and centrifugal force described in A. Choice B incorrectly describes how gravity works; the gravitational force from the cloud is essentially spherical before it flattens. Choice D describes a process that happens much later in solar system formation (the T-Tauri phase) and is responsible for clearing the disk, not forming it.

Question 2

The nebular theory explains the compositional gradient observed in our solar system's planets. Which physical process is the most direct and fundamental cause of the distinction between the inner rocky planets and the outer gas/ice giants?

The Sun's early, strong solar wind, which blew all the lighter gaseous elements to the outer solar system, leaving only heavy elements behind in the inner system.
Gravitational sorting of planetesimals after their formation, where denser, rock/metal bodies were pulled inward by the Sun's gravity while lighter, icy bodies drifted outward.
The higher density of the inner disk, which caused rocky planets to form and differentiate much faster than the planets in the less-dense outer disk.
The temperature gradient of the protoplanetary disk, which dictated that only refractory materials could condense as solids in the hot inner disk, while volatile ices could also condense in the cold outer disk. (correct answer)

Explanation: When tackling questions about planetary formation, focus on the fundamental physical conditions that existed in the early solar system and how they directly influenced what materials could exist where. The nebular theory centers on a crucial concept: the temperature gradient in the protoplanetary disk. As you move outward from the proto-Sun, temperatures dropped dramatically. This temperature difference determined which materials could condense into solid particles at different distances. In the hot inner regions (within about 3-4 AU), only refractory materials like silicates and metals could remain solid—volatile compounds like water, methane, and ammonia stayed gaseous. Beyond the "frost line" in the cooler outer disk, these volatiles could freeze into ices, providing abundant solid material for planet building. This is why answer D correctly identifies the temperature gradient as the fundamental cause of our solar system's compositional structure. Answer A incorrectly suggests solar wind was the primary sorting mechanism, but this occurred after the basic compositional pattern was already established by condensation. Answer B describes gravitational migration, which is a secondary process that may have occurred after initial formation, not the fundamental cause of compositional differences. Answer C focuses on disk density variations affecting formation speed, but this doesn't explain why different materials are found in different regions. Remember this key principle: in planetary formation questions, temperature-controlled condensation is usually the primary process that determines composition, while other mechanisms like solar wind and gravitational sorting are secondary effects that modify an already-established pattern.

Question 3

Uranus and Neptune are classified as 'ice giants' rather than 'gas giants' like Jupiter and Saturn, as they are composed of a much smaller fraction of hydrogen and helium. Within the framework of the core accretion model, what is the most plausible explanation for this difference?

Uranus and Neptune formed much farther out in the protoplanetary disk where the density of material, both solid and gas, was much lower, leading to slower growth. (correct answer)
The region where Uranus and Neptune formed was composed almost exclusively of ices, lacking the rocky material needed to form a core massive enough to attract large amounts of gas.
They originally formed as gas giants but migrated through a region of high stellar radiation that stripped away most of their hydrogen and helium atmospheres.
Jupiter and Saturn's formation consumed nearly all the available hydrogen and helium in the solar nebula, leaving little for Uranus and Neptune to accrete.

Explanation: The correct answer is A. In the outer reaches of the solar nebula where Uranus and Neptune formed, the density of material was lower and orbital periods were longer. This meant that the process of building a solid core through accretion was much slower than for Jupiter and Saturn. By the time their cores grew massive enough to begin accreting gas in a runaway fashion, the young Sun had entered its T-Tauri phase and had already begun to blow away most of the primordial hydrogen and helium from the disk. They started accreting gas too late and didn't have enough time to become as massive as Jupiter and Saturn. B is incorrect; they have rocky cores. C is implausible given their great distance from the Sun. D is an oversimplification; while Jupiter and Saturn accreted vast amounts, the primary constraint for Uranus and Neptune was their own slower growth rate.

Question 4

In the nebular theory, the formation of solid bodies begins with two key processes: condensation and accretion. Which statement accurately distinguishes these two processes in the context of the early protoplanetary disk?

Condensation is the formation of solid particles directly from the nebular gas, while accretion is the subsequent growth of these particles through collisions and sticking. (correct answer)
Accretion is the process of forming a planetary core, while condensation is the process of capturing a gaseous atmosphere around that core.
Condensation describes the formation of icy particles outside the frost line, while accretion describes the formation of rocky particles inside the frost line.
Accretion is the initial clumping of dust due to gravitational instability, while condensation is the later growth into planetesimals due to electrostatic forces.

Explanation: The correct answer is A. Condensation is a phase transition where microscopic solid particles form directly from the cooling gas of the nebula (like frost forming on a window). Accretion is the process that follows, where these tiny condensed particles stick together (first through electrostatic forces, then through gravity) and grow into larger objects like planetesimals. B incorrectly defines both terms in the context of giant planet formation. C incorrectly limits the processes to certain regions; both condensation and accretion occur on both sides of the frost line, just with different materials condensing. D incorrectly defines the mechanisms and reverses the order of key forces involved in accretion.

Question 5

The asteroid belt is located in a region between Mars and Jupiter where nebular theory might predict a planet could have formed. What is the leading explanation for why a full-sized planet did not form in this location?

There was an insufficient amount of primordial material in that specific orbital band of the nebula to begin with.
The solar wind from the young Sun was particularly strong in this region, blowing away solid particles before they could accrete.
The region was too close to the frost line, meaning the constant freezing and thawing of ices prevented stable accretion.
Jupiter's immense gravity exerted strong perturbations, increasing collision velocities and preventing planetesimals from accreting into a larger body. (correct answer)

Explanation: The correct answer is D. The most widely accepted explanation is the gravitational influence of Jupiter. As Jupiter, the most massive planet, formed, its gravity 'stirred up' the planetesimals in the nearby asteroid belt. This resulted in orbital resonances and increased the relative velocities of the planetesimals. Instead of gently colliding and sticking together (accretion), they collided at high speeds, leading to fragmentation or ejection from the region. This prevented the formation of a single large planet. A is incorrect; models suggest there was enough mass initially. B is incorrect as the solar wind primarily affects gas, not kilometer-sized planetesimals. C is incorrect as this is not a recognized mechanism for preventing accretion.

Question 6

Condensation is the formation of solid particles directly from the nebular gas, while accretion is the subsequent growth of these particles through collisions and sticking. (correct answer)
Accretion is the process of forming a planetary core, while condensation is the process of capturing a gaseous atmosphere around that core.
Condensation describes the formation of icy particles outside the frost line, while accretion describes the formation of rocky particles inside the frost line.
Accretion is the initial clumping of dust due to gravitational instability, while condensation is the later growth into planetesimals due to electrostatic forces.

Question 7

Planetary differentiation is the process by which a planet's interior separates into layers of different density (e.g., core, mantle, crust). According to the nebular theory, what were the two primary sources of heat that enabled this process in early terrestrial protoplanets?

Heat from the central protostar and tidal forces from Jupiter.
The decay of short-lived radioactive isotopes and the kinetic energy from planetesimal impacts. (correct answer)
Gravitational potential energy released during gas accretion and magnetic induction from the Sun.
Compression from the overlying layers of material and chemical reactions in the planetary core.

Explanation: The correct answer is B. For differentiation to occur, a planet's interior must become at least partially molten. The two main heat sources in the early solar system that could achieve this for terrestrial planets were: 1) The heat from accretion, where the kinetic energy of impacting planetesimals is converted into thermal energy, and 2) The decay of short-lived radioactive isotopes (like Aluminum-26) trapped within the planet's interior. A is incorrect because solar heat primarily affects the surface, and tidal forces were not the main driver for planetary differentiation. C describes heat sources more relevant to gas giants. D is a factor, but the initial heat from accretion and radioactivity was necessary to start the melting process.

Question 8

The 'angular momentum problem' was a significant historical challenge to nebular hypotheses. In the context of our solar system, what is the core issue that modern versions of the theory must successfully explain?

Why the planets all orbit in the same direction, which seems too orderly for a process starting from a random cloud.
How the initial, non-rotating interstellar cloud acquired enough angular momentum to flatten into a disk in the first place.
Why the outer gas giants orbit so much more slowly than the inner terrestrial planets, despite having more angular momentum.
How the Sun, which contains 99.8% of the system's mass, ended up with only about 1% of the system's total angular momentum. (correct answer)

Explanation: When studying planetary formation, the angular momentum problem represents one of the most elegant puzzles in astronomy. Angular momentum must be conserved in any rotating system, so when you analyze where it ended up in our solar system, something seems dramatically wrong at first glance. The core issue is a striking imbalance: our Sun contains 99.8% of the solar system's mass but holds only about 1% of its total angular momentum. The remaining 99% resides in the orbiting planets, especially the massive outer giants. This creates a fundamental puzzle because if the solar system formed from a contracting, rotating nebular disk, you'd expect the central object (the Sun) to spin much faster and retain most of the angular momentum as it concentrated most of the mass. Looking at the wrong answers: A misunderstands the problem entirely—planets orbiting in the same direction actually supports nebular theory and isn't problematic. B incorrectly suggests the issue is acquiring initial rotation, but interstellar clouds naturally have some rotation from galactic motion and gravitational interactions. C confuses orbital mechanics with the angular momentum problem—outer planets orbit more slowly because of Kepler's laws, and this is expected behavior. Modern nebular theory solves this puzzle through mechanisms like magnetic braking, where the young Sun's magnetic field couples with the surrounding disk, transferring angular momentum outward while matter flows inward. Remember: angular momentum problems in astronomy often involve unexpected distributions between central and orbiting objects—always consider where the "spin" actually ended up versus where you'd naively expect it.

Question 9

Radiometric dating of primitive meteorites, such as carbonaceous chondrites, consistently yields an age of approximately 4.56 billion years. Why is this specific age considered to be the formation age of the solar system and a cornerstone of the nebular theory?

This is the age of the oldest rocks found on Earth's surface, setting a baseline for when the planet became solid.
This age marks the time when the Sun ignited nuclear fusion, and the meteorites are debris from this event.
These meteorites contain the first solid materials to condense from the solar nebula and have remained largely unchanged since, dating the start of accretion. (correct answer)
This corresponds to the end of the late heavy bombardment, marking the point when the solar system became stable.

Explanation: The correct answer is C. Carbonaceous chondrites are considered pristine relics from the formation of the solar system. They were never part of a large, differentiated body, so their composition reflects the raw material of the solar nebula. The age of their oldest components (calcium-aluminum-rich inclusions) dates the moment when the first solid grains condensed out of the cooling nebular gas. This event marks the very beginning of the planet formation process (condensation followed by accretion), and is therefore taken as the 'birth date' of the solar system. A is incorrect; the oldest Earth rocks are younger due to geological processing. B is incorrect as the age dates the formation of solids, not the Sun's ignition. D is incorrect as the LHB happened much later (~3.9 billion years ago).

Question 10

The T-Tauri phase is a stage in the evolution of a young star that is critical for the final architecture of its planetary system. What is the primary role of this phase in shaping the solar system according to the nebular theory?

It marks the ignition of nuclear fusion in the protostar, providing the heat that establishes the temperature gradient and frost line in the disk.
It is a period of intense magnetic activity and strong stellar winds that expels the remaining gas and dust from the protoplanetary disk. (correct answer)
It triggers the rapid accretion of gas onto giant protoplanets, marking the main phase of gas giant formation before the gas dissipates.
It causes the late heavy bombardment by gravitationally destabilizing the orbits of remaining planetesimals in the system.

Explanation: The correct answer is B. The T-Tauri phase occurs after the protostar has formed and initiated fusion, but before it settles onto the main sequence. This phase is characterized by extreme stellar winds and high UV radiation. This outflow of energy and matter is the primary mechanism responsible for clearing away the primordial gas and dust that remains in the protoplanetary disk after the planets have largely formed. A is incorrect because the frost line is established earlier, during the main disk phase. C is incorrect because the T-Tauri phase ends the gas accretion phase by removing the gas, it does not trigger it. D describes the late heavy bombardment, which is thought to be caused by planetary migration, a separate (though potentially overlapping) process.

Question 11

The heavily cratered surfaces of the Moon and Mercury testify to a period of 'late heavy bombardment' (LHB) that occurred ~3.9 billion years ago, long after the planets had formed. How do modern extensions of the nebular theory, such as the Nice model, account for this event?

The LHB was caused by the Sun's T-Tauri wind clearing the final, large planetesimals from the inner solar system, causing them to collide with the planets.
The LHB represents the final stage of terrestrial planet accretion, where the Moon was formed by a giant impact with Earth.
The LHB was triggered by a shift in the orbits of Jupiter and Saturn, which destabilized the asteroid and Kuiper belts, sending a shower of bodies into the inner solar system. (correct answer)
The LHB was caused by the solar system passing through a dense spiral arm of the Milky Way, which introduced a large population of interstellar impactors.

Explanation: The correct answer is C. The Nice model, a widely accepted extension of the nebular theory, proposes that the giant planets formed in a more compact configuration than they are in today. Gravitational interactions caused their orbits to shift over hundreds of millions of years. A key event was Jupiter and Saturn crossing a 2:1 orbital resonance, which dramatically altered their orbits and, in turn, the orbits of Uranus and Neptune. This planetary migration destabilized the primordial asteroid and Kuiper belts, flinging vast numbers of planetesimals and comets into the inner solar system, causing the late heavy bombardment. A is incorrect as the T-Tauri phase happens much earlier. B is incorrect as the Moon-forming impact is thought to have happened much earlier than the LHB peak. D is incorrect as the source of the impactors is believed to be native to our solar system.

Question 12

During the formation of planets in the protoplanetary disk, the dominant mechanism for growth evolves as objects become larger. Which statement correctly describes the transition of forces responsible for binding particles together?

Gravity is the dominant force at all scales, from the initial clumping of dust grains to the formation of full-sized planets.
Initially, electrostatic forces cause small dust grains to stick together, but once bodies grow to about a kilometer in size, their self-gravity becomes the dominant force for further accretion. (correct answer)
Chemical bonds are responsible for the initial growth into centimeter-sized pebbles, after which gravitational collapse takes over to form planetesimals directly.
Initially, gravitational attraction to the protostar causes particles to collide, and then magnetic forces bind the particles together into larger aggregates.

Explanation: The correct answer is B. In the early stages, microscopic dust grains are too small for their mutual gravity to be effective. They collide and stick together due to weak electrostatic forces (like static cling). This process builds up objects to the size of pebbles and boulders. Once an object grows to a significant size (roughly 1 km, a planetesimal), its own gravitational field becomes strong enough to effectively attract and capture other nearby objects, and gravity-driven accretion becomes the dominant growth mechanism. A is incorrect because gravity is negligible for small dust grains. C is partially plausible but the transition to gravity is the key step. D incorrectly identifies the forces; the protostar's gravity governs orbits, not clumping, and magnetism is not the primary binding force.

Question 13

The discovery of 'hot Jupiters'—gas giant exoplanets orbiting extremely close to their stars—posed a puzzle for the original nebular theory. Which modification or related concept best explains the existence of these planets?

These planets formed in situ from a protoplanetary disk that was unusually dense and hot, allowing for direct gravitational collapse of gas close to the star.
These planets are failed stars that formed as part of a binary system and had their outer layers stripped away by the primary star.
These planets formed beyond the frost line, where abundant ices allowed a massive core to grow, and then migrated inward due to interactions with the gas disk. (correct answer)
These planets formed as rocky super-Earths and subsequently pulled gas directly from the star's atmosphere, growing into gas giants.

Explanation: The correct answer is C. It is too hot for ices to condense close to a star, making it extremely difficult to form a massive core via accretion in that region. The leading hypothesis is that these planets formed in the outer part of their protoplanetary disks, beyond the frost line, in the conventional way. Then, gravitational interactions between the massive planet and the remaining gas in the disk caused the planet to lose angular momentum and spiral inward, a process called planetary migration. A is inconsistent with the temperature requirements for core accretion. B and D describe mechanisms that are not supported by current models of planet formation.

Question 14

The formation of the regular moons of Jupiter (Io, Europa, Ganymede, Callisto) is often described as a 'miniature solar system.' How does this formation process, according to nebular theory, parallel the formation of the solar system itself?

Jupiter's gravity captured four large planetesimals that happened to pass by at the same time, settling them into co-planar orbits.
Interstellar dust was funneled by Jupiter's magnetic field into a ring, which then collapsed to form the four moons.
The moons are fragments of a larger body that collided with Jupiter and were thrown into orbit around it.
As Jupiter grew massive, it formed its own flattened accretion disk of gas and dust, from which the moons condensed and accreted in succession. (correct answer)

Explanation: When you encounter questions about moon formation around gas giants, think about how large planetary systems can create their own smaller versions of the original solar nebula formation process. The correct answer is D because Jupiter's formation directly parallels how our solar system formed. As Jupiter accumulated mass from the solar nebula, it became so massive that it created its own gravitational influence strong enough to gather surrounding gas and dust into a flattened, rotating disk around itself - essentially a "mini solar nebula." Within this disk, the same processes that formed the planets occurred on a smaller scale: material condensed, collided, and accreted to form the regular moons. This explains why Io, Europa, Ganymede, and Callisto all orbit in the same plane and direction, just like the planets around the Sun. Option A incorrectly suggests capture of independent bodies, but captured objects typically have irregular, tilted orbits - not the perfectly organized system we observe. Option B misunderstands the role of magnetic fields; while Jupiter's magnetosphere is powerful, it doesn't control large-scale dust distribution for moon formation. Option C describes a collision scenario that would create irregular debris patterns, not the systematic size progression and circular orbits we see in Jupiter's regular moons. Remember this key pattern: when you see regular, co-planar moon systems around gas giants, think "mini solar system formation." The organized nature of these systems is the telltale sign of disk accretion rather than random capture or collision events.

Question 15

Imagine a hypothetical protoplanetary disk that has the same mass and temperature profile as our own, but is composed of a significantly higher proportion of refractory elements (metals, silicates) relative to volatile elements (ices). What would be the most likely outcome for planet formation in such a system?

The terrestrial planets would likely be more massive, and giant planet formation beyond the frost line would be less efficient due to smaller cores. (correct answer)
The frost line would shift significantly closer to the star, allowing gas giants to form where Earth is now.
Fewer but larger planets would form overall, as the excess refractory material would accrete into a single massive body in the inner system.
Gas giants would form much more rapidly, as the rocky cores could reach critical mass without the need for ice.

Explanation: The correct answer is A. A higher proportion of refractory materials means more 'building blocks' for rocky planets are available in the hot, inner part of the disk. This would likely lead to the formation of more massive terrestrial planets (e.g., 'super-Earths'). Conversely, beyond the frost line, while there is more rock/metal, the relative lack of abundant ice—a key ingredient for rapidly building massive cores—would make it harder for protoplanets to reach the ~10 Earth mass threshold needed to accrete gas efficiently. This would make giant planet formation less efficient or result in smaller cores. B is incorrect; the frost line's location is determined by temperature, not composition. C is speculative; more material doesn't necessarily mean one single body forms. D is incorrect; even with more rock, the total mass of solids is dominated by ices beyond the frost line, so lacking them is a major hindrance to rapid core growth.

Question 16

The increased rotational speed generates an outward centrifugal force that most strongly opposes gravity along the rotational equator, allowing collapse to proceed more rapidly along the poles. (correct answer)
Gravitational forces are weakest along the rotational poles due to the cloud's spin, causing the cloud to collapse preferentially into a disk shape.
Inelastic collisions between infalling particles systematically cancel out motions perpendicular to the average plane of rotation, forcing material into a disk.
The central protostar's early solar wind exerts pressure that pushes material away from the poles more effectively than from the dense equatorial region.

Question 17

Consider a protoplanetary disk forming around a star that is significantly less luminous and cooler than the Sun. How would the 'frost line' in this system most likely compare to the one in our solar system, and what would be a probable consequence for planet formation?

The frost line would be farther from the star, leading to the formation of gas giants at much larger orbital radii than Jupiter.
The frost line would be closer to the star, potentially allowing gas giants to form at orbital radii similar to that of Mars or the asteroid belt. (correct answer)
The frost line's location would be unchanged because it depends on the density of the nebula, not the star's luminosity.
The frost line would be closer to the star, but this would result in smaller gas giants because less material would be available overall.

Explanation: The correct answer is B. The frost line is the distance from the central star where it is cold enough for volatile compounds like water to condense into solid ice. A less luminous (cooler) star radiates less energy, so this condensation temperature would be reached much closer to the star. The ability for ices to form adds a significant amount of solid material to the disk, allowing for the rapid formation of large planetary cores (~10 Earth masses) that are necessary to gravitationally attract huge amounts of hydrogen and helium gas. Therefore, a closer frost line would allow the process of gas giant formation to begin at smaller orbital radii. A is incorrect because a fainter star means a closer frost line. C is incorrect because the frost line is primarily determined by temperature, which is a function of stellar luminosity. D is incorrect because a closer frost line makes more solid material available for core-building in that region, which would facilitate the formation of large gas giants, not smaller ones.

Question 18

It marks the ignition of nuclear fusion in the protostar, providing the heat that establishes the temperature gradient and frost line in the disk.
It is a period of intense magnetic activity and strong stellar winds that expels the remaining gas and dust from the protoplanetary disk. (correct answer)
It triggers the rapid accretion of gas onto giant protoplanets, marking the main phase of gas giant formation before the gas dissipates.
It causes the late heavy bombardment by gravitationally destabilizing the orbits of remaining planetesimals in the system.

Question 19

According to the core accretion model, gas giants like Jupiter must have formed relatively quickly, within the first 3-10 million years of the solar system's existence. What is the primary reason for this critical time constraint?

After this period, the protoplanetary disk cools to a point where hydrogen and helium gas can no longer be gravitationally captured by a protoplanet's core.
The protoplanet core must reach a critical mass of ~10 Earth masses before the young Sun enters its T-Tauri phase and disperses the nebular gas. (correct answer)
The orbits of the giant planets must stabilize before the terrestrial planets begin to form, a process that takes several million years.
After a few million years, the remaining planetesimals in the outer solar system are ejected, halting the growth of the solid core needed to attract gas.

Explanation: The correct answer is B. The core accretion model requires a two-step process: 1) build a solid core of rock and ice to about 10 Earth masses, and 2) use that core's gravity to rapidly accrete a massive envelope of hydrogen and helium gas. This second step is only possible while there is still abundant gas in the protoplanetary disk. After a few million years, the young star enters its T-Tauri phase, characterized by strong stellar winds that blow away the remaining gas. Therefore, there is a race against time for the core to grow large enough to capture its atmosphere before the gas is gone. A is incorrect; cooling would actually help gas accretion. C misstates the relative timelines. D is incorrect because the primary constraint is the disappearance of the gas, not the solid building blocks.

Question 20

The LHB was caused by the Sun's T-Tauri wind clearing the final, large planetesimals from the inner solar system, causing them to collide with the planets.
The LHB represents the final stage of terrestrial planet accretion, where the Moon was formed by a giant impact with Earth.
The LHB was triggered by a shift in the orbits of Jupiter and Saturn, which destabilized the asteroid and Kuiper belts, sending a shower of bodies into the inner solar system. (correct answer)
The LHB was caused by the solar system passing through a dense spiral arm of the Milky Way, which introduced a large population of interstellar impactors.